The field of the invention is that of flat printed antennae for the transmission and/or reception of microwave signals.
More precisely, the invention relates to a flat antenna producing maximum radiation at low elevations.
The antenna of the invention has numerous applications. It can, for example, be used in a network positioned on the roof of a particular vehicle so as to provide telecommunications via satellite. In effect, certain mobile stations, and particularly those linked to geostationary satellites in countries of medium or high latitude (Northern Europe for example) require flat antennae producing maximum radiation at low elevations.
At present, for reasons of the space occupied and for cost reasons, "patch" type printed antennae are used in mobile stations. In effect, these have the notable advantage of being flat and inexpensive.
In a general way, a printed antenna includes a dielectric substrate plate, an earth plane (constituted by a first conductor layer deposited on a first face of the dielectric substrate plate), a radiating element (constituted by a second conductor layer, deposited on a second face of the dielectric substrate plate) and means of feeding the antenna.
In their everyday operation, that is to say, when they are operating in their fundamental mode, these antennae generate a radiation pattern having a maximum in the direction perpendicular to the plane that contains the antenna. For this type of everyday operation, the length of the radiating element is very close to the half wavelength taking into account the permittivity of the dielectric substrate used.
In order to be able to generate radiation having a maximum for low elevations, that is to say in directions a long way from the perpendicular axis to the plane containing the antenna, the printed antennae must operate in a higher mode, the current distribution of which allows this type of radiation to be created.
The major problem rests in the fact that the higher modes that are of interest appear for frequencies that are relatively high in relation to those of the fundamental mode. This means that in order to be able to use this type of (higher) mode for the desired frequency band (close to that corresponding to the fundamental mode), the antenna must be overdimensioned to a very high degree.
This oversizing makes it quasi-impossible to integrate it into a network of radiating elements so as to obtain high gain antennae. This size problem is all the more crucial for a network having to generate radiation at low elevation, the radiating elements must be positioned very close to one another so as to avoid large network lobes which seriously reduce the gain of the antenna.